1. Field of the Invention
The present invention relates to circuits which control supplying electricity to a load, and more particularly to circuits which limit the amount of power consumed by a load.
2. Description of the Related Art
Many types of electrical apparatus are powered by direct current. The direct current may be derived from a battery or from an alternating to direct current power supply. For example, the power supply converts 120 volt or 250 volt alternating current from a utility company into a desired lower DC voltage level compatible with the electronic circuitry.
The voltage potential of these current sources can vary in magnitude for a number of reasons. For instance, the output voltage level of an AC to DC power supply can fluctuate with variation of the AC voltage into the power supply. The output voltage also can vary due to changes in the amount of current being drawn by loads attached to the power supply. The output voltage from a battery decreases as the charge on the battery depletes and can increase while the battery is recharging.
It often is beneficial to limit the amount of electric current that is allowed to enter or flow through electrical equipment. Limiting the amount of electric current to below a defined level can prevent damage to the equipment. Such current limiting also may prevent a failure of one component from causing damage to other components.
Current limiting circuits are well known and commonly detect voltage across a sensing resistor through which all or a defined portion of the electric current to a load flows. The level of the sensed voltage is in proportion to the magnitude of the electric current. That sense voltage level is compared to a reference voltage level that corresponds to the desired current limit. The result of that comparison can be used to control the flow of current to the load. For example, if the reference voltage level is exceeded, indicating an excessively high current flowing to the load, a disconnect device can be activated to terminate that current flow and prevent damage to the load.
Under certain circumstances it is desirable to limit the input power to a circuit to less than a prescribed wattage level prescribed. For example, equipment can be designed with greater latitude for clearances between conductors and other parameters of the circuit layout, when the equipment draws less then 200 volt-amperes. Conventional design standards are more stringent for equipment that may consume more electrical power. Therefore, it is desirable to ensure that this power level is not exceeded so that a less expensive and complicated circuit layout can be utilized in the equipment.
However, merely regulating the voltage or current applied to the apparatus does not ensure that the circuitry will not draw in excess of the desired power limit. For example, if the apparatus is nominally powered at 15 volts and consumes 150 watts of power, its input current is 10 amps. A power supply that limits the current level to 10 amps could allow the apparatus to consume more than a 150 watts when the input voltage rises above 15 volts. In this example if the supply voltage is greater than 20 volts, a design guideline of 200 volt-amperes is exceeded. As noted previously, such supply voltage variation is not uncommon, especially among battery powered equipment, which can vary significantly depending upon the charge level of the battery. For example, it is not unusual for a battery voltage to be relatively high when a battery charger is active and then decrease to about half that voltage level when battery is the discharged.
Therefore, it is desirable to provide a reliable, inexpensive circuit which limits the power consumption of an electrical load to less than a predefined level.